This invention relates generally to processing image data acquired by a system having volumetric computed tomography (VCT) capability, and more particularly, to expanding the field-of-view of a CT scanner.
The current hardware of the volumetric CT (VCT) scanner has a reconstruction field-of-view (FOV) that is limited to 50 cm. Although sufficient for many clinical applications, it is desirable to expand the FOV to image objects currently outside the FOV. This is particularly advantageous for applications such as oncology and multi-modality imaging, such as CT combined with at least one of positron emission tomography (PET) and single photon emission computed tomography (SPECT) capability. For example, in multi-modality imaging using a CT/PET system, the FOV of the PET system may be larger than the FOV of the CT system. In some applications, it is desirable to have a consistent FOV between the CT and the other (PET or SPECT) imaging system.
By way of example, for oncology applications, a larger FOV may be desirable as the limbs of the patient are often positioned outside the FOV of the CT detector for better tumor positioning during radiation treatment planning. The current VCT reconstruction algorithm ignores the truncated projections (data outside the FOV) and produces images with severe artifacts which may negatively affect an accurate estimation of the attenuation path for treatment planning.
Combined PET/CT and SPECT/CT scanners obtain inherently registered PET (or SPECT) and CT images, which are used to produce a fused image. For example, the PET emission data is corrected for the attenuation of the 511 keV photons (or positrons) using an attenuation map derived from the CT image. The VCT images have a smaller FOV (50 cm) than the PET scanner (70 cm), resulting in missing attenuation data from structures outside 50 cm. CT artifacts resulting from truncated data propagate into the PET image and result in corruption. Therefore, expanding the VCT FOV to match the PET FOV is highly desirable.
Missing projections have been compensated for in various ways. Many of the corrections rely on the consistency condition of the measured projection data to estimate the missing projection. For example, for a parallel beam geometry, the total amount of attenuation of a single projection does not change over different angles. When the cone angle is small, such as approximately 1 degree for a 16 slice CT scanner, this assumption is approximately valid.
However, the consistency condition does not work well in cone beam configuration where the cone angle may be approximately 4 degrees or larger. The divergent nature of the cone beam can no longer be ignored. For a 16 slice CT scanner, a plot of total attenuation as a function of a view results in a relatively straight line. For VCT, however, the plot is an odd shaped curve and can no longer be used to determine where truncation takes place or what area of an object is outside the FOV. For example, as the scanner rotates around the patient, outer rows with axial angles of more than two degrees measure attenuation through a changing section of the patient. Thus the plot of total attenuation per projection around the patient is modulated by the axial variation in the patient, making it difficult to detect variations which may be caused by truncation of the transaxial FOV.
Therefore, a need exists for increasing the FOV coverage of a CT scanner. Certain embodiments of the present invention are intended to meet these needs and other objectives that will become apparent from the description and drawings set forth below.